print((function (f, s)
	local h,t={},0
	for i=1,13 do
		local c=f(s,i,i)
		if h[c]==nil then h[c],t=0,t+1 end h[c]=h[c]+1
		if h[c]>2 or t>7 then return "Impossible" end
	end
	for k,v in pairs(h) do if v==1 then return k end end
end)(string.sub, io.stdin:read("*l")))